How can I have two users on the same machine share a Photos Library?
I want to the users on my family iMac to share the Photos library, so that we only need to have one copy on the machine.
iMac, iOS 8.3
To give others selective access to your Photos or iPhoto library, you have the option of using iCloud Photo Sharing, if the privacy implications don't bother you. The images will be stored on Apple servers.
If you want to give full read/write access to more than one user, see the support article linked below.
iPhoto: Sharing libraries among multiple users
There is a way to share the library without moving it to a secondary volume. If you really need to do that, ask for instructions.